1. Field of the Invention
The present invention relates to a lithium secondary battery and, more particularly, to a lithium secondary battery, in which electrolyte impregnation capability is improved to enhance the mobility of the electrolyte.
2. Description of the Prior Art
Recently, a large number of compact and lightweight electrical/electronic appliances, such as cellular phones, notebooks, computers, etc., have been developed and produced. These electrical/electronic appliances are provided with battery packs so that they can operate in places without separate power sources. A battery pack includes at least one battery capable of outputting a given level of voltage to drive an electrical/electronic appliance for a certain period of time.
Due to economical efficiency reasons, a battery pack has been adapted to employ a rechargeable secondary battery. Non-limiting examples of secondary batteries include a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium secondary battery, such as a lithium battery and a lithium ion battery.
In particular, since the lithium secondary battery has an operating voltage of 3.6V, which is three times higher than that of the nickel-cadmium battery or the nickel-hydrogen battery, and has a high energy density per unit weight, the demands for the lithium secondary battery have been rapidly expanded.
A lithium secondary battery may use lithium-based oxides as positive electrode active materials, and uses carbon materials as negative electrode active materials. In general, the lithium secondary battery may be classified into either a liquid electrolyte battery or a polyelectrolyte battery according to the type of the electrolyte used. A lithium secondary battery using the liquid electrolyte is referred to as a lithium ion battery, and a lithium secondary battery using the polyelectrolyte is referred to as a lithium polymer battery. The lithium secondary battery may also be fabricated into various shapes. According to the shapes, the lithium secondary battery may be a cylinder type battery, a square type battery, or a pouch type battery.
The cylinder type lithium secondary battery usually includes an electrode assembly. The electrode assembly includes a positive electrode plate coated with positive electrode active materials, a negative electrode plate coated with negative electrode active materials, and a separator positioned between the positive electrode plate and the negative electrode plate to prevent a short circuit between the two electrode plates and to allow only lithium ions to pass through. The positive electrode plate, the negative electrode plate, and the separator of the electrode assembly are rolled up in an approximately cylindrical shape and inserted into a cylindrical case, and an electrolyte is injected into the cylindrical case to enable the lithium ions to migrate.
Before the electrode assembly is inserted into the cylindrical case, a bottom insulator plate is inserted first so as to provide insulation between the electrode assembly and the cylindrical case. Also, after the electrode assembly is inserted but before the cylindrical case is sealed with a cap assembly, a top insulator plate is inserted so as to provide insulation between the electrode assembly and the cap assembly.
Similarly, in a square type lithium secondary battery, an insulator case (or plate) for supporting a cap assembly and providing insulation between a terminal plate and an electrode assembly, and a bottom insulator plate for providing insulation between a square case and the electrode assembly are inserted into the square case.
Since bottom insulator plates are usually made of polyethylene (PE) or polypropylene (PP), they have no affinity with an electrolyte. This hinders the electrolyte from being sufficiently impregnated into an electrode assembly. In the structure of an existing bottom insulator plate with a center pin, a central hole may be formed to abut one end of the center pin so that there is not enough room for impregnating the electrolyte into the electrode assembly. In the case of a battery not having the center pin, no hole is formed. Further, the bottom insulator plate is in close contact with the electrode assembly, which also acts as a hindrance to the impregnation of the electrode assembly with the electrolyte.
Moreover, as current trends are moving toward high capacity batteries, an electrode assembly is becoming more dense, and thus its outer diameter is becoming larger. As the outer diameter of the electrode assembly becomes larger, a space between a case and the electrode assembly is reduced, which makes it more difficult to impregnate an electrolyte into the electrode assembly. As such, there is a need to form a bottom insulator plate in such a manner that the electrolyte can be easily impregnated into the electrode assembly.